This invention relates to apparatus and methods for locating magnetic objects or sources. The term "locating" as used hereinafter is intended to include the tracking of magnetic objects. The term "magnetic objects" as used hereinafter is intended to include bodies or sources that have an intrinsic magnetic field, as well as objects that produce a detectable perturbation in an extrinsic magnetic field, such as the earth's magnetic field.
The prior art is replete with apparatus for locating magnetic objects such as surveyor's benchmarks, underground utility lines, and buried artifacts. Both active locating apparatus, which generates a magnetic field and senses modification of the field by the object to be detected, and passive locating apparatus, which merely senses a magnetic field associated with the object, are well known. Apparatus of the latter type includes vector gradiometers which sense the magnetic field gradient. Because of the many possible combinations of sources, orientations, polarity, and strength of magnetic fields associated with physical objects, unique determination of the position of a magnetic object without prior knowledge of the magnetic properties of the object has been difficult.